Within the context of the present disclosure, the expression “internal combustion engine” encompasses Otto-cycle engines, diesel engines and also hybrid internal combustion engines, which utilize a hybrid combustion process, and hybrid drives which comprise not only the internal combustion engine but also an electric machine which may be connected in terms of drive to the internal combustion engine and which receives power from the internal combustion engine or which, as a switchable auxiliary drive, additionally outputs power.
Supercharging serves primarily to increase the power of the internal combustion engine. The air required for the combustion process is compressed, as a result of which a greater air mass can be supplied to each cylinder per working cycle. In this way, the fuel mass and therefore the mean pressure may be increased.
Supercharging is a suitable means for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. Supercharging leads to an increase in volumetric power output and an improved power-to-weight ratio. If the swept volume is reduced, it is thus possible to shift the load collective toward higher loads, at which the specific fuel consumption is lower. By means of supercharging in combination with a suitable transmission configuration, it is also possible to realize so-called downspeeding, with which it is likewise possible to achieve a lower specific fuel consumption. Supercharging consequently assists in the constant efforts in the development of internal combustion engines to minimize fuel consumption to improve the efficiency of the internal combustion engine.
For supercharging, an exhaust-gas turbocharger including a compressor and a turbine are arranged on the same shaft. The hot exhaust-gas flow is supplied to the turbine and expands in the turbine with a release of energy, which sets the shaft in rotation. The energy supplied by the exhaust-gas flow to the turbine and ultimately to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor delivers and compresses the charge air supplied to it, as a result of which supercharging of the cylinders is obtained.
A charge-air cooler may be provided in the intake system downstream of the compressor, cooling the compressed charge air before it enters at least one cylinder. The charge-air cooler lowers the temperature and thereby increases the density of the charge air, improving charging of the at least one cylinder.
The exhaust-gas turbocharger, for example, in comparison with a mechanical charger, may utilize the exhaust-gas energy of the hot exhaust gases for transmitting power between the charger and internal combustion engine and may not need a mechanical connection. In contrast, a mechanical charger extracts the energy required for driving it entirely from the internal combustion engine, and thereby reduces the output power and consequently adversely affects the efficiency.
Problems are encountered in the configuration of the exhaust-gas turbocharging, wherein it is basically sought to obtain a noticeable performance increase at all engine speed ranges. In the case of supercharged internal combustion engines with an exhaust-gas turbocharger, a noticeable torque drop is observed when a certain engine speed is undershot. The effect is undesirable and is thus, also one of the most severe disadvantages of exhaust-gas turbocharging.
The torque drop is understandable if one takes into consideration that the charge pressure ratio is dependent on the turbine pressure ratio. For example, if the engine speed is reduced, this leads to a smaller exhaust-gas mass flow and therefore to a lower turbine pressure ratio. As a result, the charge pressure ratio likewise decreases in the direction of lower engine speeds, which equates to a torque drop.
According to the prior art, a variety of measures have been used to improve the torque characteristic of an exhaust gas-turbocharged internal combustion engine, including a small turbine cross section and simultaneous provision of an exhaust-gas blow-off facility. To this end, the turbine is equipped with a bypass line which branches off from the exhaust-gas discharge system upstream of the turbine and in which a shut-off element is arranged. Such a turbine is also referred to as a wastegate turbine. If the exhaust-gas mass flow exceeds a critical value, a part of the exhaust-gas flow is conducted past the turbine, that is to say is blown off, via a bypass line during the course of the so-called exhaust-gas blow-off. This procedure has the disadvantage that the high-energy blown-off exhaust gas remains unutilized and the supercharging behavior is often insufficient at higher engine speeds.
A turbine having a variable turbine geometry permits a more comprehensive adaptation to the respective operating point of the internal combustion engine by way of adjustment of the turbine geometry or the effective turbine cross section, enabling engine speed-dependent or load-dependent regulation of the turbine geometry to take place to a certain extent.
The torque characteristic of the supercharged internal combustion engine may also be improved by means of multiple turbochargers arranged in parallel, for example, by means of multiple turbines of relatively small turbine cross section arranged in parallel. The turbines may be activated successively with increasing exhaust-gas flow rate, similar to sequential supercharging.
The torque characteristic may also be influenced by connecting multiple exhaust-gas turbochargers in series. In one example, connecting two exhaust-gas turbochargers in series, wherein a first exhaust-gas turbocharger serves as a high-pressure stage and a second exhaust-gas turbocharger serves as a low-pressure stage, the compressor characteristic map may be expanded to include both smaller compressor flows and larger compressor flows.
In particular, with the first exhaust-gas turbocharger, which serves as a high-pressure stage, it is possible for the surge limit to be shifted in the direction of smaller compressor flows, because of which high charge pressure ratios may be obtained even with small compressor flows, which may considerably improve the torque characteristic in the lower part-load range. This is achieved by using the high-pressure turbine for small exhaust-gas mass flows and by providing a bypass line by means of which, with increasing exhaust-gas mass flow, an increasing amount of exhaust gas is conducted past the high-pressure turbine. For this purpose, the bypass line branches off from the exhaust-gas discharge system upstream of the high-pressure turbine and opens into the exhaust-gas discharge system again downstream of the high-pressure turbine and upstream of the low-pressure turbine, that is to say between the two turbines, wherein a shut-off element is arranged in the bypass line in order to control the exhaust-gas flow conducted past the high-pressure turbine.
The two exhaust-gas turbochargers connected in series further increase the power boost through supercharging. Furthermore, the response behavior of an internal combustion engine with two exhaust-gas turbochargers may be considerably improved, particularly in the part-load range compared to a similar internal combustion engine with single-stage supercharging. The reason for this is that the relatively small high-pressure stage is less inert than a relatively large exhaust-gas turbocharger used for single-stage supercharging, because a rotor or impeller of an exhaust-gas turbocharger of smaller dimensions may accelerate and decelerate more quickly.
This also has advantages with regard to particle emissions. In a large single exhaust-gas turbocharger, during acceleration, the required increase in the air mass supplied to the cylinders for the increased fuel flow rate takes place only with a delay owing to the inertia of the large impellers. In contrast, with a relatively small high-pressure turbocharger, the charge air is supplied to the engine virtually without a delay, and thus operating states with increased particle emissions are more commonly eliminated.
Exhaust-gas turbocharging in combination with exhaust-gas aftertreatment has proven to be problematic. When using an exhaust-gas turbocharger, it is fundamentally sought to arrange the turbine of the charger as close to the engine, that is to say to the outlet openings of the cylinder, as possible in order thereby to be able to optimally utilize the exhaust-gas enthalpy of the hot exhaust gases, which is determined significantly by the exhaust-gas pressure and the exhaust-gas temperature, and to ensure a fast response behavior of the turbocharger. Furthermore, the path of the hot exhaust gases to the different exhaust-gas aftertreatment systems should also be as short as possible such that the exhaust gases are given little time to cool down and the exhaust-gas aftertreatment systems reach their operating temperature or light-off temperature as quickly as possible, in particular after a cold start of the internal combustion engine.
The thermal inertia of the part of the exhaust lines situated between the outlet opening at the cylinder and the turbine, or between the outlet opening at the cylinder and the exhaust-gas aftertreatment system, should therefore also be as low as possible, which may be achieved by reducing the mass and the length of the corresponding parts.
To achieve the above-stated aims, in one example, exhaust lines may be substantially merged within the cylinder head. The length of the exhaust lines is reduced by way of the integration into the cylinder head, whereby not only the thermal inertia but also the line volume of the relevant part are reduced, improving the response behavior of a turbine, and increasing the enthalpy of the exhaust gases at the inlet into the turbine.
While a single turbine may easily be positioned close to the engine, arranging a plurality of turbines close to the engine simultaneously may be a problem, for example, as in the internal combustion including two turbines arranged in series.
EP 1 396 619 A1 relates to the simultaneous use of exhaust-gas turbocharging and exhaust-gas aftertreatment, wherein the exhaust-gas aftertreatment system may be arranged as close as possible to the outlet of the internal combustion engine. In one embodiment according to EP 1 396 619 A1, the exhaust-gas flow may be conducted past both turbines by means of a suitable switching device and bypass line. This offers advantages with regard to a catalytic converter arranged in the exhaust-gas discharge system downstream of the turbines, in particular after a cold start or in the warm-up phase of the internal combustion engine, because the hot exhaust gases are supplied directly to the catalytic converter and are not firstly conducted through the turbines which are to be regarded as a temperature sink. In this way, the catalytic converter reaches its light-off temperature more quickly after a cold start or in the warm-up phase. A further embodiment provides the arrangement of a second catalytic converter, for example, a primary catalytic converter in the bypass line that bypasses the two turbines.
However, the inventors herein have recognized potential issues with such systems. As one example, a disadvantage of the concept described in EP 1 396 619 A1 is that in the warm-up phase of the internal combustion engine, all of the exhaust gas is supplied to the at least one catalytic converter for heating purposes, and no exhaust gas is conducted through the turbines, such that, during the warm-up phase, no supercharging occurs owing to a lack of charge pressure.
US 2009/0178406 A1 and US 2012/0216529 A1 describe an internal combustion in which an exhaust-gas aftertreatment system is arranged between the turbines. A bypass line, which branches off from the exhaust-gas discharge system upstream of the high-pressure turbine, opens into the exhaust-gas discharge system again upstream of the low-pressure turbine, having bypassed the high-pressure turbine and the said exhaust-gas aftertreatment system. In the presence of low exhaust-gas flow rates, in particular after a cold start or in the warm-up phase, the exhaust gas is supplied to the small high-pressure turbine, whereby supercharging of the internal combustion engine is achieved under these operating conditions. In the references cited above, the exhaust-gas aftertreatment system arranged downstream of the high-pressure turbine serves to ensure the required conversion of the pollutants.
With increasing exhaust-gas flow, an increasing amount of exhaust gas is conducted past the high-pressure turbine, increasing amounts of untreated exhaust gas flowing to the low-pressure turbine. Therefore, it may be desirable to have an additional exhaust-gas aftertreatment system downstream of the low-pressure turbine. However, the use of noble metals is primarily responsible for the high production costs of an exhaust-gas aftertreatment system. The need to provide more than one exhaust-gas aftertreatment system may increase costs significantly along with packaging disadvantages due to the increased space requirement of the exhaust-gas aftertreatment arrangement. The stated conflict between exhaust-gas turbocharging and exhaust-gas aftertreatment cannot be resolved according to the prior art.
In one embodiment, a supercharged internal combustion engine may include an intake system for the supply of charge air to at least one cylinder and having an exhaust-gas discharge system for the discharge of the exhaust gas from the at least one cylinder and having at least two exhaust-gas turbochargers which are arranged in series. A first exhaust-gas turbocharger may serve as a low-pressure stage and a second exhaust-gas turbocharger serve as a high-pressure stage. A second turbine of the second exhaust-gas turbocharger may be arranged in the exhaust-gas discharge system upstream of a first turbine of the first exhaust-gas turbocharger. A second compressor of the second exhaust-gas turbocharger may be arranged in the intake system downstream of a first compressor of the first exhaust-gas turbocharger. A first bypass line may branch off from the exhaust-gas discharge system upstream of the second turbine and may join back the exhaust-gas discharge system again to form a junction point between the first turbine and the second turbine with a shut-off element. An exhaust-gas recirculation arrangement may be coupled to the supercharged internal combustion engine. At least one exhaust-gas aftertreatment system may be arranged in the exhaust-gas discharge system between the first turbine and the second turbine.
This arrangement of the exhaust-gas aftertreatment system has the technical effect that all of the exhaust gas, under all operating conditions, passes entirely through the at least one exhaust-gas aftertreatment system, such that no further additional exhaust-gas aftertreatment system of the same type has to be provided, that is to say is necessary, downstream of the low-pressure turbine. This therefore yields advantages with regards to both, cost and packaging.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.